This application addresses broad Challenge Area (11) Regenerative Medicine and specific Challenge Topic, 11-HL-101* Develop cell-based therapies for cardiovascular, lung, and blood diseases. Given their self-renewing and pluripotent nature, human embryonic stem (hES) and induced pluripotent stem (iPS) cells hold great promise as cell replacement therapy and regenerative medicine. We recently developed a 2D culture system for differentiating hES cells into the endothelial lineage, and demonstrated that hES cell-derived endothelial cells (hES-EC) can self-assemble into functional blood vessels that spontaneously integrate into the circulatory system after transplantation into immunodeficient mice. However, a key challenge for clinical translation of hES-EC-based therapies is the immunological rejection. The major goal of this project is to mechanistically understand the immunological responses to allogeneic hES-EC, and to develop effective approaches to preventing the rejection of hES cell-derived endothelial allografts. A key factor currently impeding the study of the immunogenicity and allogenicity of hES/iPS cells and their derivatives is the lack of a suitable in vivo model. We have developed a novel protocol for human immune reconstitution in immunodeficient mice. The humanized mice (hu-mice) exhibit sustained repopulation with multilineages of human lymphohematopoietic cells and formation of secondary lymphoid organs, develop strong antigen-specific human T cell and antibody responses upon immunization, and mediate transplant rejection. In this proposal, we will use this hu-mouse model to pursue the following two specific aims. In Aim 1, we will examine the allogenicity of hES-EC in hu-mice. The goal of this aim is to understand the allogenicity of hES-EC and determine whether hES-EC at different differentiation stages (e.g., early vs. late hES-EC) differ in allogenicity. First, we will determine the allogenicity of hES-EC using a series of in vitro assays. We will then assess in vivo immune responses to allogeneic hES-EC in hu-mice. We will also assess the survival/rejection of hES-EC allografts in hu-mice that are depleted of human T cells or T cell subset(s) for identifying the immune components that are essential to hES-EC allograft rejection. In Aim 2, we will explore the possibility of inducing tolerance to hES-EC allografts in hu-mice through establishment of mixed hematopoietic chimerism. We will develop protocols for in vitro differentiation of transplantable hematopoietic stem/progenitor cells from hES cells, and with these cells to induce chimerism and donor-specific tolerance in hu-mice. These studies will determine whether hES-EC derived from the same hES cell line that is used for establishing hematopoietic chimerism can be permanently accepted by the chimeric hu-mice without the need of immunosuppression. Successful completion of the proposed studies will help us to understand the mechanisms of alloimmune responses to hES-EC and develop effective strategies to overcome the immunological hurdle to hES cell-based regenerative medicine. PUBLIC HEALTH RELEVANCE: Immunological rejection is a key challenge for clinical translation of human embryonic stem (hES) and induced pluripotent stem (iPS) cell-based regenerative medicine. This proposal aims to use a humanized mouse model to assess the allogenicity of hES cell-derived endothelial cells and to develop strategies for controlling alloimmune responses to hES cell-derived endothelial allografts.